Metal catalyst composition modified by nitrogen-containing compound

ABSTRACT

The present invention relates to a metal catalyst composition modified by a nitrogen-containing compound, which effectively reduces cathode catalyst poisoning. The catalyst composition applied on the anode also lowers the over-potential. The catalyst coupled with the nitrogen-containing compound has increased three-dimensional hindrance, which improves the distribution of the catalyst particles and improves the reaction activity.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan applicationserial no. 99146629, filed Dec. 29, 2010 and the priority benefit ofTaiwan application serial no. 100103141, filed Jan. 27, 2011. Theentirety of each of the above-mentioned patent applications is herebyincorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a catalyst. Moreparticularly, the present invention relates to a metal catalyst for thefuel cell.

2. Description of Related Art

On account of the great demand for clean energy, fuel cells have beenwidely applied in industry, housing and transportation. Direct methanolfuel cell (DMFC) directly converts methanol into electric energy forportable applications, including laptops and mobile phones.

In principle, methanol that diffuses to the cathode (methanol crossover)will cause catalyst poisoning in the cathode, especially when using highconcentrations of methanol as the fuel. Also, high overpotential lossesof the anode exist in the direct methanol fuel cell, leading to lowerdischarge voltage and inferior activity.

SUMMARY OF THE INVENTION

The present invention is directed to a metal catalyst modified by anitrogen-containing compound. Taking advantage of the coordination ofthe d orbital of the metal catalyst with the lone pairs of the nitrogenatoms in the nitrogen-containing compound, the binding positions of themetal catalyst for carbon mono-oxide (CO) are occupied and becomeunavailable, which effectively reduces cathode catalyst poisoning. Thecatalyst composition applied on the anode also lowers the overpotential.The steric hindrance of the metal catalyst coupled with thenitrogen-containing compound is increased, which improves the dispersionof the catalyst particles and increases the reaction activity.

The present invention provides a catalyst composition applicable on theelectrode surface of a fuel cell. The catalyst composition includes ametal catalyst and a nitrogen-containing compound. Thenitrogen-containing compound is a nitrogen-containing 5-memberedheterocyclic compound. The nitrogen-containing 5-membered heterocycliccompound can be an unsaturated, substituted or non-substituted,nitrogen-containing 5-membered ring compound.

According to embodiments of the present invention, thenitrogen-containing compound has the structure of the following formula(I):

wherein R₁-R₅ can individually be hydrogen, alkyl, NH₂ or NR₆R₇, R₆ andR₇ can individually be alkyl, hydroxyl or alkoxyl, X₁, X₂, X₃, X₄ and X₅can individually be carbon or nitrogen but at least one of X₁, X₂, X₃,X₄ and X₅ is nitrogen.

According to embodiments of the present invention, thenitrogen-containing compound has the structure of the following formula(II):

wherein S₁-S₇ can individually be hydrogen, alkyl, NH₂ or NS₈S₉, S₈ andS₉ can individually be alkyl, hydroxyl or alkoxyl, Y₁, Y₂, Y₃, Y₄ and Y₅can individually be carbon or nitrogen but at least one of Y₁, Y₂, Y₃,Y₄ and Y₅ is nitrogen.

The present invention provides a membrane electrode assembly applicablefor the fuel cells. The membrane electrode assembly includes a protonexchangeable polymer membrane, a cathode catalyst layer and an anodecatalyst layer respectively disposed on both sides of the polymermembrane and two gas diffusion layers respectively disposed on thecathode and anode catalyst layers. At least one of the cathode and anodecatalyst layers includes a catalyst composition having a metal catalystand a nitrogen-containing compound, which compound is anitrogen-containing 5-membered heterocyclic compound.

According to embodiments of the present invention, thenitrogen-containing compound has the structure of the following formula(I):

wherein R₁-R₅ can individually be hydrogen, alkyl, NH₂ or NR₆R₇, R₆ andR₇ can individually be alkyl, hydroxyl or alkoxyl, X₁, X₂, X₃, X₄ and X₅can individually be carbon or nitrogen but at least one of X₁, X₂, X₃,X₄ and X₅ is nitrogen.

According to embodiments of the present invention, thenitrogen-containing compound has the structure of the following formula(II):

wherein S₁-S₇ can individually be hydrogen, alkyl, NH₂ or NS₈S₉, S₈ andS₉ can individually be alkyl, hydroxyl or alkoxyl, Y₁, Y₂, Y₃, Y₄ and Y₅can individually be carbon or nitrogen but at least one of Y₁, Y₂, Y₃,Y₄ and Y₅ is nitrogen.

According to embodiments of the present invention, thenitrogen-containing compound can be selected from the following groupconsisting of pyrrole and pyrrole derivatives (pyrroles), pyrroline andpyrroline derivatives (pyrrolines), imidazole and imidazole derivatives(imidazoles), imidazoline and imidazoline derivatives (imidazolines),and triazole and triazole derivatives (triazoles). The metal catalystcan be a pure metal catalyst or a metal catalyst with a support. Theweight ratio of the active portion of the metal catalyst (excluding thesupport) and the nitrogen-containing compound ranges from 1:2 to 25:1.

In order to make the above and other features and advantages of thepresent invention more comprehensible, embodiments accompanied withfigures are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a schematic display of the membrane electrode assembly.

FIG. 2 is a voltammogram showing the oxygen reduction current versuspotential for Pt—C cathode.

FIG. 3 is a cyclic voltammogram showing the current versus potential forPt/C cathode.

FIGS. 4( a)-4(b) are voltammograms showing the current versus potentialfor Pt—C cathode at the first cycle and the 20^(th) cycle of the ORRmeasurement with methanol presented.

Common reference numerals are used throughout the drawings and thedetailed description to indicate the same elements. The presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings.

DESCRIPTION OF THE EMBODIMENTS

The present invention is directed to a metal catalyst modified by anitrogen-containing compound. Taking advantage of the coordination ofthe d orbital of the metal catalyst with the lone pairs of the nitrogenatoms in the nitrogen-containing compound, the binding positions of themetal catalyst for carbon mono-oxide (CO) are occupied and becomeunavailable, which effectively reduces cathode catalyst poisoning.

The metal catalyst coupled with the nitrogen-containing compound hasincreased steric hindrance, which improves the dispersion of thecatalyst particles, lowers the activity loss from aggregation andincreases the reaction activity. The catalyst composition applied on theanode also lowers the overpotential.

The present invention provides a catalyst composition including a metalcatalyst modified by a nitrogen-containing compound. The weight ratio ofthe active portion of the metal catalyst (excluding the support) and thenitrogen-containing compound ranges from 1:2 to 25:1.

The metal catalyst herein includes all of the common metal catalystsapplicable for various fuel cells, including the pure metal catalyst orthe metal catalyst with the support. The materials of the pure metalcatalyst include Pt, Pt—Ru, Pt—Co, Pt—Rh, Pt—Sn, Pt—Ni and Pt—Au, thecombinations and the alloys thereof, for example. The support can becarbon supports, including carbon black, carbon nanotubes, porous carbonand sea urchin shaped carbon nanostructured materials, etc. Thematerials of the catalyst for the present invention are not limited bythe above-mentioned examples, and the modification or adjustments of themetal catalyst and the support are encompassed within the scope of thisinvention.

The catalyst composition of this invention is applicable on theelectrode surface of a fuel cell. The nitrogen-containing compound ofpresent invention is a nitrogen-containing 5-membered heterocycliccompound. The nitrogen-containing 5-membered heterocyclic compound canbe an aromatic or non-aromatic, nitrogen-containing 5-membered ringcompound.

According to one embodiment of the present invention, thenitrogen-containing compound has the structure of the following formula(I):

wherein R₁-R₅ can individually be hydrogen, alkyl, NH₂ or NR₆R₇, R₆ andR₇ can individually be alkyl, hydroxyl or alkoxyl, or anelectron-pushing group, X₁, X₂, X₃, X₄ and X₅ can individually be carbonor nitrogen but at least one of X₁, X₂, X₃, X₄ and X₅ is nitrogen.

According to another embodiment of the present invention, thenitrogen-containing compound has the structure of the following formula(II):

wherein S₁-S₇ can individually be hydrogen, alkyl, NH₂ or NS₈S₉, S₈ andS₉ can individually be alkyl, hydroxyl or alkoxyl, Y₁, Y₂, Y₃, Y₄ and Y₅can individually be carbon or nitrogen but at least one of Y₁, Y₂, Y₃,Y₄ and Y₅ is nitrogen.

The nitrogen-containing compound of the present invention can beprepared based on the commonly used preparation methods or synthesisprocesses in the organic chemistry field. Depending on the varioussubstitutional groups, the solvents, reaction parameters, reactiontemperatures or additives can be accordingly adjusted or changed. Suchadjustments should not be limited to the embodiments provided herein andis well understood by the artisan.

The catalyst of this invention can be prepared by the common preparationmethods, including colloid, micro-emulsion, and impregnation methods.

Taking the catalyst with carbon black support and prepared by thecolloid method as an example, anhydrous metal salts is added to the drysolvent under waterless nitrogen environment and the reducing agent isslowly added to the mixture solution dropwise. When the solution turnsblack or hydrogen gas is generated, the reduction reaction is completeand stably dispersed as a colloid solution. The solution is stirred andcarbon black is added as the catalyst support, keeps stirring to removethe solvent, washed by ethanol and dried up to obtain the metalcatalyst. The catalyst prepared by the colloid method has small particlesizes of about 1.5 nm to 3 nm and good dispersion. The most commonimpregnation method is to dissolve the metal salts in the solvent,impregnate with carbon black under stirring and reduced by adding thereducing agent. Based on the preparation conditions, the catalystprepared by the impregnation method may be changeably dispersed and haveparticle sizes ranging from 2 nm to 20 nm.

The metal catalyst of this invention can be applicable for fuel cellsand other fields, such as electrochemical batteries, including airbatteries.

In general, the fuel cell includes electrodes, separation/exchangemembrane and current collectors. The oxidation reaction of the fuel andthe reduction reaction of the oxidizing agent occur by the electrodes(the anode and the cathode), and the membrane functions to separate theoxidizing agent and the reducing agent and allows the protons to passthrough.

For direct methanol fuel cells (DMFC), methanol is oxidized intohydrogen ions (protons) and electrons that travel through the externalcircuit as the electric output of the fuel cell. The hydrogen ionstravel through the membrane and react with oxygen from the cathode andthe electrons from the external circuit to form water and generate heat.The electrodes set of DMFC is a membrane electrode assembly.

FIG. 1 is schematic display of the membrane electrode assembly of thefuel cell. Referring to FIG. 1, the membrane electrode assembly 10includes a separation/polymer membrane 100. The separation/polymermembrane 100 can be a proton exchange membrane (PEM) of a solid ionicpolymer material, such as, Nafion ionic polymer membrane. Theseparation/polymer membrane 100 must allow proton transport and preventelectron and gas moving across. The catalyst reaction layers 110 on bothsides of the separation/polymer membrane 100 include an anode catalystlayer 110A and a cathode catalyst layer 110C upon where theelectrochemical reactions of the anode and the cathode occur. The gasdiffusion layers 120 are disposed on the catalyst reaction layers 110(respectively on the anode catalyst layer 110A and the cathode catalystlayer 110C). The material of the anode catalyst layer 110A may be PtRuon carbon support (PtRu/C), while the material of the cathode catalystlayer 110C may be Pt/C, for example. The material of the electrode(either anode or cathode) may be Pt/C, PtRu/C, PtRu or Pt, for example.

The exemplary preparation method of the catalyst composition includesmixing the metal salts, the carbon support (such as, carbon black),perfluorocarbon sulfonic acid solution, alcohol/water mixture and thenitrogen-containing compound of the present invention under ultrasonicvibration and stirring, so as to prepare the catalyst slurry. Later, thecatalyst slurry is coated to the gas diffusion layers by the doctorblade to form the gas diffusion electrodes and vacuum dried. Theresultant gas diffusion electrodes (anode and cathode) and the protonexchange membrane are thermally pressed together, in the sequence ofanode-PEM-cathode, so as to obtain the membrane electrode assembly.

In the following experiments, the fuel cell electrodes (cathode oranode) using the metal catalyst modified by the nitrogen-containingcompound of the present invention are compared with the fuel cellelectrode using the unmodified metal catalyst.

The performances of the catalyst composition can be evaluated bymeasuring electrochemical surface active area (ECSA), methanol oxidationreaction (MOR), and oxygen reduction reaction (ORR). In thesemeasurements, the evaluated electrodes are prepared by coating thecatalyst slurry to the glassy carbon disc of the rotation disc electrode(RDE), and then vacuum dried at 60° C. for 2 hours. The catalyst slurryis prepared by mixing the platinum catalyst, the nitrogen-containingcompound, Nafion solution (from DuPont) and solvent(s) under stirringfor better dispersion and de-bubbling.

The electrochemical surface area (ECSA) of fuel cell electrodes can bemeasured by CV analysis, presented as a voltammogram of current vs. theworking electrode potential, involving cycling the evaluated electrodeover a potential range where charge transfer reactions areadsorption-limited at the activation sites. The ECSA of the catalyst orcatalyst composition is calculated from the charge obtained from the CVexperiment.

Integration of the hydrogen desorption/adsorption peaks that result as aconsequence of the forward and reverse scans, respectively, is used toestimate the electrochemically active surface area of the electrocatalyst.

ECSA: The electrolytic solution (0.5M sulfuric acid) is supplied withnitrogen gas to remain oxygenless, and the measurement is performedunder room temperature. Cyclic voltammetry (CV) scanning is performed ata scan rate of 5 mV/s, the potential cycling from 1.0V to 0V (vs.reversible hydrogen electrode (RHE)) and under the electrode rotationspeed of 0 rpm. The recorded spectrum is analyzed to determine theintegration area of the hydrogen adsorption peak (three platinum crystalfaces from 0.05V to 0.3V) in the reduction scan (scanning from highvoltage to low voltage) for comparison.

MOR: The measurement is similar to ECSA, except the electrolyticsolution is (0.5M sulfuric acid+1M methanol). The recorded spectrum isanalyzed to determine the integration area of the methanol oxidationcurrent (scanning from low voltage to high voltage) of the electrode(s)(from 0.3V to 0.7V), which is directly proportional to the activity ofMOR. The onset potential for the oxidation peak represents the onset ofmethanol oxidation.

ORR: After supplying the electrolytic solution (0.5M sulfuric acid) withoxygen for 30 minutes, the RDE is placed into the solution and theoxygen supply remains. The electrolytic solution will be added with 0.1Mmethanol, if the methanol tolerance property is measured. Themeasurement is performed under room temperature. Cyclic voltammetry (CV)scanning is performed at a scan rate of 5 mV/s, the potential cyclingfrom 1.1V to 0.5V (vs. RHE) and under the electrode rotation speed of1600 rpm. The recorded spectrum is analyzed to determine current valueat 0.6V, which is reversely proportional to the reaction activity (i.e.the negative value is proportional to the reduction current). For themethanol tolerance experiment (Exp 4), the methanol oxidation peakvalues at 0.6V-0.8V are compared, which is reversely proportional tomethanol tolerance (larger the positive value means larger the oxidationcurrent and less the methanol tolerance). Small methanol tolerance meansthat the catalyst activity is easily affected by methanol. The oxidationpeaks appear in later cycles indicate better tolerance to methanol (i.e.less prone to methanol poisoning).

Exp 1: Anode catalyst composition using PtRu/C modified with variousnitrogen-containing compounds of the present invention.

The measurements of MOR are performed with the electrolytic solution(0.5M sulfuric acid+1M methanol) and the metal catalyst with thesupport, where 70% wt of the total weight of the metal catalyst with thesupport (PtRu/C) is the metal catalyst. That is, the metal catalystportion of the metal catalyst with the support (PtRu/C) takes 70% wt ofthe total weight of the metal catalyst composition. The weight ratio ofthe metal catalyst portion to the nitrogen-containing compound is about1:1.6. The control group uses the unmodified catalyst composition having70% wt PtRu/C, which lacks of the nitrogen-containing compound.

TABLE 1 MOR MOR oxidation oxidation Overpotential peak potential peakcurrent Samples Onset (V) E_(peak) (V) I_(peak) (A/g) Control: 70%PtRu/C 0.294 0.73 210 Sample 1: 70% PtRu/C+ 0.275 0.73 425 (modified by2,4 dimethyl-2-imidazoline) Sample 2: 70% PtRu/C+ 0.293 0.788 394(modified by imidazole) Sample 3: 70% PtRu/C+ 0.277 0.74 196 (modifiedby pyridine) Sample 4: 70% PtRu/C+ 0.286 0.76 359 (modified by triazole)

From Table 1, compared with the control sample, it is shown that themetal catalysts modified by the nitrogen-containing compound of thepresent invention have lower overpotentials (about 10-20 mV lower) andhigher MOR oxidation peak currents (increased to about 1.5 to 2 times),and thus have increased activities. By using the nitrogen-containingcompound(s) of the present invention to modify PtRu/C as the anodecatalyst, the anode overpotential is lowered and the activity of theanode metal catalyst is improved by the catalyst particles coupled withthe nitrogen-containing compound(s). For the nitrogen-containingcompounds of Table 1, the five-membered heterocyclic compounds workbetter and pyridine has little effects when compared with the control.

Exp 2: Anode catalyst composition using PtRu/C and various ratios of thenitrogen-containing compound (triazoles).

The measurements of MOR are performed with the electrolytic solution(0.5M sulfuric acid+1M methanol) and the metal catalyst with thesupport, where the metal catalyst portion of the metal catalyst with thesupport (PtRu/C) takes 70% wt of the total weight of the metal catalystcomposition. The weight ratio of the metal catalyst portion to thenitrogen-containing compound ranges from about 2.5:1 to about 30:1.

TABLE 2 Samples Onset (V) I_(peak) (A/g) 70% PtRu/C 0.294 270 (withoutthe nitrogen-containing compound) 70% PtRu/C+ the nitrogen-containingcompound 0.29 378 (addition ratio 2.5:1) 70% PtRu/C+ thenitrogen-containing compound 0.275 436 (addition ratio 10:1) 70% PtRu/C+the nitrogen-containing compound 0.277 379 (addition ratio 20:1) 70%PtRu/C+ the nitrogen-containing compound 0.284 267 (addition ratio 30:1)

From Table 2, compared with the control (without the nitrogen-containingcompound), it is found that the samples with the addition ratios from2.5:1 to 20:1 provide increased catalyst activities and lowered anodeoverpotentials. However, the sample with the addition ratio of about10:1 works better.

Exp 3: Cathode catalyst composition using nitrogen-containing compoundsand Pt/C (the addition ratio of the metal catalyst tonitrogen-containing compound (imidazolines)=4:1).

The measurements of ORR are performed. Using the acidic solutionsupplied with oxygen, cyclic voltammetry (CV) scanning is performed withthe potential cycling from 1.1V to 0.5V (vs. RHE). The values of thecurrent density (A/g) at the same voltage for various samples areanalyzed, which are proportional to the ECSA values of the catalysts.Referring to FIGS. 2 and 3, by adding the nitrogen-containing compoundimidazolines, the activities of the cathode catalysts are improved withthe ECSA values increased by one time and the ORR current values at 0.6V increased by 60%.

Exp 4: Cathode catalyst composition using nitrogen-containing compoundsand Pt/C (the addition ratio of the metal catalyst tonitrogen-containing compound (imidazoles)=8:1).

The anti-poisoning measurements are performed, using the ORRmeasurements plus methanol with the electrolytic solution (0.5M sulfuricacid+1M methanol). Referring to FIGS. 4( a)-(b), the electrode poisoningoccurs at later cycles for the electrodes with the metal catalystsmodified by the nitrogen-containing compounds, indicating better anti-COpoisoning capabilities. It is observed that no methanol oxidation peakshows at the first cycle of ORR. For the electrodes with the metalcatalysts modified by the nitrogen-containing compounds, the values ofthe present methanol oxidation peaks are lower (close to zero). Hence,the addition of the nitrogen-containing compound into the metal catalystcompositions indeed helps to improve the anti-poisoning ability of thecatalysts.

In the above embodiments, the direct methanol fuel cell is used as anexample of the potential applications of the nitrogen-containingcompounds of the present invention. However, the applications or detailsand conditions of the operating methods should not be limited to theembodiments provided herein and can be utilized in other related fields,structures or products that are well understood by the skilled in art.

While the invention has been described and illustrated with reference tospecific embodiments thereof, these descriptions and illustrations donot limit the invention. It should be understood by those skilled in theart that various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the invention asdefined by the appended claims. The illustrations may not be necessarilybeing drawn to scale. There may be distinctions between the artisticrenditions in the present disclosure and the actual apparatus due tomanufacturing processes and tolerances. There may be other embodimentsof the present invention which are not specifically illustrated. Thespecification and the drawings are to be regarded as illustrative ratherthan restrictive. Modifications may be made to adapt a particularsituation, material, composition of matter, method, or process to theobjective, spirit and scope of the invention. All such modifications areintended to be within the scope of the claims appended hereto. While themethods disclosed herein have been described with reference toparticular operations performed in a particular order, it will beunderstood that these operations may be combined, sub-divided, orre-ordered to form an equivalent method without departing from theteachings of the invention.

1. A catalyst composition, comprising a metal catalyst and anitrogen-containing compound, wherein the nitrogen-containing compoundis a nitrogen-containing 5-membered heterocyclic compound.
 2. Thecomposition of claim 1, wherein the nitrogen-containing 5-memberedheterocyclic compound is an unsaturated, nitrogen-containing 5-memberedring compound.
 3. The composition of claim 1, wherein thenitrogen-containing compound has the structure of the following formula(I):

wherein R₁-R₅ are individually hydrogen, alkyl, NH₂ or NR₆R₇, R₆ and R₇are individually alkyl, hydroxyl or alkoxyl, X₁, X₂, X₃, X₄ and X₅ areindividually carbon or nitrogen but at least one of X₁, X₂, X₃, X₄ andX₅ is nitrogen.
 4. The composition of claim 1, wherein thenitrogen-containing compound has the structure of the following formula(II):

wherein S₁-S₇ are individually hydrogen, alkyl, NH₂ or NS₈S₉, S₈ and S₉are individually alkyl, hydroxyl or alkoxyl, Y₁, Y₂, Y₃, Y₄ and Y₅ areindividually carbon or nitrogen but at least one of Y₁, Y₂, Y₃, Y₄ andY₅ is nitrogen.
 5. The composition of claim 1, wherein thenitrogen-containing compound is selected from the following groupconsisting of pyrrole and pyrrole derivatives (pyrroles), pyrroline andpyrroline derivatives (pyrrolines), imidazole and imidazole derivatives(imidazoles), imidazoline and imidazoline derivatives (imidazolines),and triazole and triazole derivatives (triazoles).
 6. The composition ofclaim 5, wherein the nitrogen-containing compound is selected from thefollowing group consisting of N-methylpyrrole, 1,2,3-triazole,1,2,4-triazole, 2-methyl-2-imidazoline and 2,4-dimethyl-2-imidazoline.7. The composition of claim 1, wherein the metal catalyst is a puremetal catalyst or a metal catalyst with a support.
 8. The composition ofclaim 1, wherein a weight ratio of the metal catalyst and thenitrogen-containing compound ranges from 1:2 to 25:1.
 9. A membraneelectrode assembly for a fuel cell, comprising: a proton exchangeablepolymer membrane; a cathode catalyst layer and an anode catalyst layerrespectively disposed on both sides of the polymer membrane; and two gasdiffusion layers respectively disposed on the cathode and anode catalystlayers, wherein at least one of the cathode and anode catalyst layersincludes a catalyst composition having a metal catalyst and anitrogen-containing compound, and the nitrogen-containing compound is anitrogen-containing 5-membered heterocyclic compound.
 10. The assemblyof claim 9, wherein the nitrogen-containing 5-membered heterocycliccompound is an unsaturated, nitrogen-containing 5-membered ringcompound.
 11. The assembly of claim 9, wherein the nitrogen-containingcompound has the structure of the following formula (I):

wherein R₁-R₅ are individually hydrogen, alkyl, NH₂ or NR₆R₇, R₆ and R₇are individually alkyl, hydroxyl or alkoxyl, X₁, X₂, X₃, X₄ and X₅ areindividually carbon or nitrogen but at least one of X₁, X₂, X₃, X₄ andX₅ is nitrogen.
 12. The assembly of claim 9, wherein thenitrogen-containing compound has the structure of the following formula(II):

wherein S₁-S₇ are individually hydrogen, alkyl, NH₂ or NS₈S₉, S₈ and S₉are individually alkyl, hydroxyl or alkoxyl, Y₁, Y₂, Y₃, Y₄ and Y₅ areindividually carbon or nitrogen but at least one of Y₁, Y₂, Y₃, Y₄ andY₅ is nitrogen.
 13. The assembly of claim 9, wherein thenitrogen-containing compound is selected from the following groupconsisting of pyrrole and pyrrole derivatives (pyrroles), pyrroline andpyrroline derivatives (pyrrolines), imidazole and imidazole derivatives(imidazoles), imidazoline and imidazoline derivatives (imidazolines),and triazole and triazole derivatives (triazoles).
 14. The assembly ofclaim 13, wherein the nitrogen-containing compound is selected from thefollowing group consisting of N-methylpyrrole, 1,2,3-triazole,1,2,4-triazole, 2-methyl-2-imidazoline and 2,4-dimethyl-2-imidazoline.15. The assembly of claim 9, wherein the metal catalyst is a pure metalcatalyst or a metal catalyst with a support.
 16. The assembly of claim9, wherein a weight ratio of the metal catalyst and thenitrogen-containing compound ranges from 1:2 to 25:1.